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Virtualization of distributed real-time systems enables the consolidation ofmixed-criticality functions on a shared hardware platform thus easing systemintegration. Time-triggered communication and computation can act as an enablerof safe hard real-time systems. A time-triggered hypervisor that activatesvirtual CPUs according to a global schedule can provide the means to allow fora resource efficient implementation of the time-triggered paradigm invirtualized distributed real-time systems. A prerequisite of time-triggeredvirtualization for hard real-time systems is providing access to a global timebase to VMs as well as to the hypervisor. A global time base is the result ofclock synchronization with an upper bound on the clock synchronizationprecision.  We present a formalization of the notion of time in virtualized real-timesystems. We use this formalization to propose a virtual clock condition thatenables us to test the suitability of a virtual clock for the design ofvirtualized time-triggered real-time systems. We discuss and model howvirtualization, in particular resource consolidation versus resourcepartitioning, degrades clock synchronization precision. Finally, we apply ourinsights to model the IEEE~802.1AS clock synchronization protocol and derive anupper bound on the clock synchronization precision of IEEE 802.1AS. We presentour implementation of a dependent clock for ACRN that can be synchronized to agrandmaster clock. The results of our experiments illustrate that a type-1hypervisor implementing a dependent clock yields native clock synchronizationprecision. Furthermore, we show that the upper bound derived from our modelholds for a series of experiments featuring native as well as virtualizedsetups.

Clock Synchronization in Virtualized Distributed Real-Time Systems using IEEE 802.1AS and ACRN